The prevalence of abnormalities in lung elasticity in patients with asthma
or chronic obstructive pulmonary disease (COPD) is still unclear. This
might be due to uncertainties concerning the method of analysis of
quasistatic deflation lung pressure-volume curves. Pressure-volume curves
were obtained in 99 patients with moderately severe asthma or COPD. These
patients were a subgroup from a Dutch multicentre trial; the entire group
was selected on the basis of a moderately lowered % predicted forced
expiratory volume in one second (FEV1), and a provocative concentration of
histamine producing a 20% decrease in FEV1 (PC20) < 8 mg.mL-1 obtained
with the 2 min tidal breathing technique. The curves were fitted with an
exponential (E) model and an exponential model which took the linear
appearance in the mid vital capacity range into account
(linear-exponential (LE)). The linear-exponential model showed a markedly
better fit ability, yielding additional parameters, such as the compliance
at functional residual capacity (FRC) level as slope of the linear part
(b), and the volume at which the linear part changed into the exponential
part of the curve (transition volume (Vtr)). Vtr (mean value Vtr/total
lung capacity (TLC) = 0.79 (SD 0.07)) showed a close positive linear
correlation with obstruction and hyperinflation variables, which might be
due to airway closure, already starting at elevated lung volumes. The
exponential shape factor K was closely correlated with b and mean values
(K = 1.32 (SD 0.05) kPa-1; b = 2.96 (SD 1.16) L,kPa-1) and the
relationship with age was comparable with data reported in healthy
individuals. The shape factor of the linear-exponential fit showed no
correlation with any elasticity related variable. Neither the elastic
recoil at 90% TLC, as obtained from the linear-exponential fit, nor its
relationship with age were significantly different from healthy
individuals. We conclude that, for a more accurate description of the lung
pressure-volume curve, a linear-exponential fit is preferable to an
exponential model. However, the physiological relevance of the shape
parameter (KLE) is still unclear. These results indicate that patients
with moderately severe asthma or COPD had, on average, no appreciable loss
of elastic lung recoil as compared with healthy individuals